The present invention relates to a process for producing reinforcing steel in the form of rods or rod wire with a yield point of at least 450 N/mm.sup.2, accompanied by good weldability and toughness.
An attempt is made when producing reinforcing steel to obtain higher yield points, whilst retaining good toughness and welding characteristics. In this context, the term good welding characteristics is understood to mean the suitability of such reinforcing steels for the presently conventional welding processes, such as e.g. electric arc hand welding, shielded arc welding, flash butt welding and resistance spot welding. The criterion for the evaluation of the weldability is the carbon content or carbon equivalent, whereby said values must be as low as possible.
The following reinforcing steels with high yield points are known:
1. Naturally hard reinforcing steels.
They achieve their yield point through alloying in the following alloying elements: approximately 0.4% carbon, approx. 1.2% manganese and approximately 0.5% silicon. Due to the high carbon content, these steels are not weldable. 2. Naturally hard reinforcing steels with the addition of microalloyin elements.
Limited weldability is possible in that part of the carbon is replaced e.g. by vanadium, the alloying elements having the following values: carbon approx. 0.3%, manganese approx. 1.2%, silicon approx. 0.5% and vanadium approx. 0.03%.
3. Naturally hard reinforcing steels with increased addition of microalloys and increased nitrogen contents.
Due to the strength-increasing action of the vanadium nitrides which form in an uncontrolled manner, it is possible to further reduce the carbon content, so that the steel is weldable. Such steels are e.g. described in the Union Carbide publication "CARVAN & NITROVAN", vanadium carriers produced by Union Carbide for steel production. They have the following alloying elements: carbon approx. 0.2%, manganese approx. 1.2%, silicon approx. 0.5% and vanadium approx. 0.08%. However, weldability is obtained with higher production costs resulting from the addition of vanadium.
4. Strain-hardened reinforcing steels.
These steels attain their characteristics through strain-hardening, such as e.g. twisting, stretching or drawing. From the carbon equivalent standpoint, they are weldable and have the following alloying elements: carbon equal to or below 0.2%, manganese approx. 0.6% and silicon approx. 0.2%. However, as a result of too much heat being introduced, during welding such steels can lose their strength. In addition, the additional strainhardening stage increases costs.
5. Reinforcing steels heat treated from the rolling heat.
Reinforcing steels are known (e.g. from DE-AS 2,353,034 and East German Pat. 84,615), which achieve their higher yield point, in that they are heat treated from the rolling heat during or immediately following rolling. As a result of an intense quenching in water, hardening of the surface zone of the rod is obtained and following the leaving of the cooling zone, this is retained by the heat in the rod core. Thus, the known temperature profiles are used which, due to the poor thermal conductivity of steel compared with other metals are normally obtained during the cooling or heating processes.
Due to the low carbon equivalent, similar to that of strain-hardened steel (carbon equal to or below 0.2%, manganese approx. 0.6% and silicon approx. 0.2%) such steel can be readily welded.
However, for the purpose of this process, it is necessary to have adequate cooling water quantities and space for the cooling zone in the rolling train. The surface of the rolling stock is cooled to a temperature of less than 200.degree. C. and after running up onto the cooling bed the compensating temperature is approximately 600.degree. C. Due to the low surface temperature, increased demands are made on the hot shear with respect to shearing force and blade quality and the conveying means to the cooling bed are subject to faster wear.
In addition, this cooling process cannot be performed in a satisfactory manner in the case of very high rolling speeds, such as e.g. occur with wire rolling. A further difficulty occurs on coiling, if the surface temperature is below 200.degree. C. and is then only heated up again to approximately 600.degree. C.
In this connection, it is pointed out that the use of reinforcing steel in the form of profiled rod wire in rings, particularly as a raw material for binding plants is constantly increasing.